This invention relates generally to signal processing circuitry for fiber optic rotation sensors. In particular this invention relates to apparatus and methods for using quasi non-multiplexed signal processing in a multiplexed fiber optic rotation sensor.
A fiber optic rotation sensor uses the Sagnac effect in a coil of optical fiber to detect rotations about a sensing axis that is perpendicular to the plane of the coil. Counterpropagating light waves in the sensing coil experience a phase shift that is related to the rotation rate. The phase shift in each wave is dependent upon the direction of propagation of the waves relative to the direction of rotation. The phase shift is seen as a change in the interference pattern formed when the waves are combined. The interference pattern is produced when two waves of the same polarization have traversed the fiber optic sensing coil in opposite directions and then interfere. The interference pattern may be monitored by directing it onto a photodetector, which produces an electrical signal indicative of the intensity of the light in the interference fringe pattern.
The time required for the light to travel through the spool is called .tau. time. The output light intensity (sensor output) is a function of the phase shift between the clockwise and counterclockwise traveling light. A fiber optic rotation sensor typically will include a phase modulator that is used to shift the phase of the counterpropagating waves to adjust the output signal. The total phase shift between the counterpropagating waves is generated by rotation of the coil, feed back compensation in closed loop operation, and modulation applied to the waves. Feedback and modulation are electronically generated and applied to the phase modulator.
The sensor output signal level has a cosine characteristic as a function of optical phase shift. The cosine is a slowly changing function when its argument is near zero. Hence, the absolute signal level has little sensitivity around zero phase shift. To overcome this problem phase modulation is required. Square wave modulation of n.pi./2 (n=1 or-1 or 3 or-3) will achieve operating points at half of the maximum sensor output with maximum sensitivity on the cosine characteristic (maximum slope). Modulation should be reapplied synchronously after every elapsed .tau. time interval to prevent the sensor output from going to maximum and the sensitivity from going to zero.
Some fiber optic rotation sensors are not modulated and not sampled every .tau. time. Instead, modulation is applied and the sensor output is sampled only every three .tau. times. This modulation and sampling scheme allows time for multiplexing electronics required for three fiber optic rotation sensors in a triad system to measure rotations about three mutually perpendicular axes. Multiplexing is used to save costs that would be incurred in forming three essentially identical systems. There have been difficulties in modulating the one sensor without causing undesirable modulation of one or more of the other two sensors.